1. Field of the Invention
The invention relates to repair of service-degraded superalloy component castings, such as turbine blades and vanes. More particularly the invention relates to repair or new fabrication of superalloy component castings by an electroslag welding process that re-melts a base substrate casting surface and grows it within a pool of molten filler material. As the combined melted material solidifies it forms a cast-in-place substrate extension that at least partially replicates the original substrate casting's crystal structure.
2. Description of the Prior Art
“Structural” repair of service-degraded gas turbine or other superalloy component castings is commonly recognized as replacing damaged material with matching alloy material and achieving properties, such as strength, that are close to the original manufacture component specifications (e.g., at least seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced surface cracks or blade tip erosion, so that risk of further cracking is reduced, and the blades are restored to original material structural and dimensional specifications.
Structural repair or new fabrication of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgic properties of the finished blade material. For example, a superalloy having more than 6% aggregate aluminum or titanium content, such as CM247 alloy, is more susceptible to strain age cracking when subjected to high temperature welding than a lower aluminum-titanium content X-750 superalloy. The finished turbine blade alloys are typically strengthened during post casting heat treatments which render them difficult to perform subsequent structural welding. Currently used welding processes for superalloy structural fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding rod or other filler material added, in order to repair cracks or build up eroded surfaces. When a blade constructed of such a material is welded with filler metal of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties comparable to a new component.
Alternative superalloy welding processes, including laser microcladding with chill fixtures, welding in so called “hot” boxes at elevated temperatures, and inertia friction welding may still lead to post weld heat treatment strain age cracking. Other friction welding processes, such as friction stir welding, can reduce superalloy cracking propensity, but the employed welding apparatus has relatively limited tool life. The alternative superalloy welding processes are not well-suited for rebuilding large gross volume of eroded component substrate material, such as for example rebuilding of an eroded turbine blade tip or vane.
In comparison to structural repair or fabrication, “cosmetic” repair or fabrication of superalloys is recognized as replacing damaged material (or joining two components of newly fabricated material) with unmatching alloy material of lesser structural property specifications, where the localized original structural performance is not needed. For example, cosmetic repair may be used in order to restore the repaired component's original profile geometry, including relatively mild turbine blade tip or vane erosion. As noted above, it is desirable to perform structural repairs on surface cracks in order to reduce their likelihood of subsequent spreading when the component is returned to service. Conversely, an example of cosmetic repair is for filling surface pits (as opposed to structural cracks) on a turbine blade airfoil in order to restore its original aerodynamic profile, where the blade's localized exterior surface is not critical for structural integrity of the entire blade. Cosmetic repair or fabrication is often achieved by using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and lower application temperature that does not negatively impact the superalloy substrate's material properties.
Given the shortcomings of superalloy structural repair welding, often the only commercially acceptable solution is to scrap damaged turbine blades that require structural repair, because past experience has shown limited success of such structural repairs. Thus repairs have been limited to those that have in the past been proven to be performed successfully by alternative superalloy welding processes described above, or by cosmetic welding, employing more ductile welding rod filler materials with reduced structural strength.
Thus, a need exists in the art for a method for performing structural fabrication of superalloy component castings, or a method for performing structural repairs on surfaces of service-degraded superalloy component castings, such as turbine vanes and blades, so that structural cracks, eroded surfaces and other surface defects can be repaired.
Another need exists in the art to increase successful rates of structural repairs of service-degraded superalloy component castings, such as turbine vanes and blades, so that damaged component scrap rates can be reduced.
Yet another need exists in the art for a method for performing structural fabrication of superalloy component castings, or repairs on surfaces of service-degraded superalloy component castings, such as turbine vanes and blades, that do not require complex welding or post-repair heat treatment procedures.